The present invention relates to a scanning probe microscope represented by a scanning interatomic force microscope (AFM: Atomic Force Microscope), and more particularly to a scanning probe microscope adapted to convert a surface geometry of a sample into color information depending on its surface frequency in order to effect color display.
The scanning probe microscope such as an AFM uses a cantilever provided with a probe needle at a tip of a cantilever beam in order to detect a microscopic texture or structure of a sample surface by utilizing an interaction between the sample surface and the probe.
The scanning of the probe needle utilizing such a cantilever over a sample surface causes an attractive force or a repulsive force between the sample surface and the probe needle on the basis of interatomic force. Consequently, if this interatomic force is detected as a cantilever strain amount and a sample stage is slightly moved in a Z direction so as to make this strain amount constant, that is, so as to make a gap between the sample surface and the probe needle constant, a slight-movement signal thereof or a detected strain amount itself will represent a geometry of the sample surface.
FIG. 13 is a block diagram showing one example of a signal processing system of a conventional scanning probe microscope. A sample 52 is rested on a three-dimensional sample stage 55, and above the sample 52 there is oppositely arranged a probe needle 54 fitted at a free end of a cantilever 53. The strain amount in the cantilever 53 is detected by measuring, using a position detector 73, an incident position of a laser beam 72 output by a laser generator 71.
The position detector 73 is constituted by a four-segment light detecting electrode, and aligned in position such that a spot of a laser beam 72 comes to a center of the four-divided electrode when the strain amount of the cantilever 53 is 0. Accordingly, if a strain occurs on the cantilever 53, the spot of a laser beam 72 moves over the four-segment electrode, thereby producing a difference in the voltage output by the four-segment electrode. This difference in voltage is amplified by a differential amplifier 74 and input as a strain signal S1 representative of a gap between the sample surface and the probe needle 54 to a non-inverted terminal (+) of a comparator 75. The comparator 75 has an inverted input terminal (-) to which a target value signal as to the strain amount in the cantilever 53 is input from a target value setting section 79.
An error signal S2 output by from the comparator 75 is input to a proportional integration (PI) control section 76. From the PI control section, a resultant signal of the error signal S2 and its integration value is input, as an observed image signal S3 and also as an actuator slight-movement signal for controlling the gap between the sample surface and the probe needle 54 to a predetermined value, to an amplifier 81 and an actuator driving amplifier 70.
A scan signal generating section 78 supplies a slight-movement signal for slightly moving the sample 52 in XY directions to the actuator driving amplifier 70. The position detector 73, the differential amplifier 74, the comparator 75, the PI control section 76 and the actuator drive amplifier 70 constitute a feedback circuit.
The observed image signal S3 is appropriately amplified by the amplifier 81 and thereafter supplied to an A/D converter 82 where it is converted into image data and stored in an image memory 83. An image memory control section 84 outputs an address signal and a lead signal to the image memory 83 in synchronism with a clock signal output by a synchronous signal generator 85. The image data output by the image memory 83 in response to the address signal and the lead signal is supplied to a RAM-DAC 86. The RAM-DAC 86 converts the image data into an analog signal in response to horizontal and vertical synchronizing signals, and the converted image data is output to a monitor unit 87.
Where a roughness of a sample surface is to be monochromatically displayed with accuracy, a gradation representation of approximately 16 bits is ideally required. The monitor unit 87, however, is low in gradation representability. In addition, the increase in gradation requires an increase in the resolving power of the A/D converter 82 or the memory capacity of the image memory 83, thereby resulting in expensive apparatus cost. To avoid this, the above-stated prior art apparatus is typically designed to represent each pixel concentration with 8 bits (64 gradations), so that there has been a problem that the sample surface roughness cannot be accurately represented.
In order to solve such problem, there has been proposed a method in which image data is put into a computer and the data is subjected to image processing so as to convert it into a three-dimensional representation. However, since the image data processing requires a high-speed processor and a large-capacity image memory, there has been the problem that the apparatus becomes expensive as well.
Further, in the above-stated prior art apparatus, if the space frequency as to the sample surface roughness is high, and the probe needle 1 is comparatively quick in scan speed, the probe needle 1 cannot follow the roughness as the case may be. Where the feedback circuit is insufficient in gain, the comparator 75 outputs an error signal S2 depending upon a difference between the strain signal S1 and the target value. The PI control section 76 outputs an observed image signal S3 as an actuator slight-movement signal in order to effect feedback control for approximating the error signal to zero. However, the error signal S2 cannot be reduced completely to zero. Consequently, the observed image signal S3 always becomes insufficient in signal component corresponding to the error signal S2, thereby resulting in bluntness at its edge portion.
In order to solve such problem, there has been a proposed structure, as in the prior art apparatus shown in FIG. 14, that is provided with a switching section 77 for selectively outputting either one of the strain signal S1 or an observed image signal S3 to the amplifying section 81 depending upon a switching signal separately input.
In the above-stated structure, however, the switching section 77 has to be switched to the strain signal S1 side when an edge portion is to be recognized with preference, while the switching section 77 switched to the observed image signal S3 side when a roughness state is to be recognized with preference. Due to this, there has been a problem in that an edge portion and a roughened portion are impossible to be recognized with accuracy at the same time.
It is an object of the present invention to provide a scanning probe microscope which is capable of converting a sample surface geometry into color information depending on its surface frequency to provide color display, thereby making it possible to accurately recognize the sample surface geometry.